The preparation of di- and polyamines of the diphenylmethane series (MDA) by reaction of aniline with formaldehyde in the presence of acidic catalysts is generally known. For the purposes of the present invention, di- and polyamines of the diphenylmethane series are understood as meaning amines and mixtures of amines of the following type:

Here, n stands for a natural number ≧2. Hereinbelow, the compounds of this type in which n=2 are referred to as diamines of the diphenylmethane series or diaminodiphenylmethanes (subsequently MMDA). Compounds of this type in which n>2 are referred to in the context of this invention as polyamines of the diphenylmethane series or polyphenylenepolymethylenepolyamines (subsequently PMDA). Mixtures of both types are referred to as di- and polyamines of the diphenylmethane series (subsequently MDA). The corresponding isocyanates, which can be derived formally from the compounds of the formula (I) by replacing all of the NH2 groups with NCO groups, are accordingly referred to as diisocyanates of the diphenylmethane series (subsequently MMDI), polyisocyanates of the diphenylmethane series or polyphenylenepolymethylenepolyisocyanates (subsequently PMDI) or di- and polyisocyanates of the diphenylmethane series (subsequently MDI). The polymer (n>2) here is usually always present both in the case of the amine as well as in the case of the isocyanate in a mixture with the dimer (n=2), meaning that in practice only two compound types are relevant, the pure dimers (MMDA or MMDI) and the mixture of dimers and polymers (MDA or MDI).
Industrially, the di- and polyamine mixtures are converted to the corresponding di- and polyisocyanates of the diphenylmethane series predominantly by phosgenation. The continuous or partially discontinuous preparation of MDA is disclosed e.g. in U.S. Pat. No. 5,286,760, EP-A-451442 and WO-A-99/40059.
The work-up of the acidic reaction mixture obtained in the preparation is initiated according to the prior art by neutralization with a base. According to the prior art, the neutralization usually takes place at temperatures of, for example, 90° C. to 100° C. without the addition of further substances (H. J. Twitchett, Chem. Soc. Rev. 3(2), p. 223 (1974)). However, it can also take place on a different temperature level in order e.g. to increase the rate of the degradation of troublesome byproducts. Hydroxides of the alkali metal and alkaline earth metal elements are suitable as bases. Aqueous NaOH is preferably used.
After the neutralization, the organic phase is separated from the aqueous phase in a separating container. The crude-MDA-containing organic phase which remains after separating off the aqueous phase is subjected to further work-up steps, such as e.g. a washing with water (base washing), in order to wash residual salts from the crude MDA. Finally, the crude MDA purified in this way is freed from excess aniline, water and other substances present in the mixture (e.g. further solvents) by means of suitable processes such as e.g. distillation, extraction or crystallization. In particular, two-stage or multistage distillations can be used. In the case of vapor/liquid separation, in the preevaporation in the case of such a two-stage or multistage distillation, the entrainment of MDA-containing droplets into the vapor phase can be reduced by a process step known to the person skilled in the art for drop separation, e.g. by incorporating gravitational separators, centrifugal separators such as e.g. cyclones, impingement separators or baffle plate separators, such as e.g. lamella separators, fixed beds or packings for drop separation, knitted-fabric separators, Venturi separators or a combination of the aforementioned mechanisms. As is known from EP 1 813 598 B1, an increased MDA fraction in the feed aniline leads to quality losses in the MDA and MDI prepared therefrom.
In the same process step, aminic secondary components in the aniline, which may also originate from the aniline process itself and which have already been described in EP 2 103 595 A1, in particular the low-boiling unsaturated and/or substituted (cyclo)aliphatic primary, secondary or tertiary amines such as cyclohexylamine, N,N-dicyclohexylamine and N-methylcyclohexylamine, can be removed. This reduces the amount of HCl required in the protonation of the aminal because these secondary components can otherwise compete with aniline during the protonation and thus reduce the effective protonation.
The work-up customary according to the prior art is disclosed for example in EP 1 652 835 A1, page 3, line 58 to page 4, line 13 and EP 2 103 595 A1, page 7, line 21 to 37. EP 1 652 835 A1 teaches that the separation off of the aqueous phase from the product-containing organic phase after the neutralization and/or the subsequent washing can be very considerably impaired by the formation of a third phase (mulm or mulm layer). This third phase is a stable, sometimes voluminous intermediate phase which arises between the aqueous and the organic phase and hinders phase separation and, in extreme cases, even prevents it completely. In the worst case for operational progression, the phase separation container or containers have to be emptied completely and cleaned. The content of the phase separation container or containers then has to be laboriously worked up or disposed of, Which is associated with considerable costs. Under certain circumstances, this can also lead to the continuous production having to be interrupted. To solve this problem, the cited specification proposes a process in which the hydrochloric acid used as catalyst comprises less than 0.001 percent by weight of divalent and/or polyvalent metal ions.
The same problem is observed in WO 2008/148631 A1, it being proposed in this case to use formalin which comprises less than 0.001 percent by weight of divalent and/or polyvalent metal ions.
In both patent applications it is not described that besides HCl and formalin, the other starting materials of the MDA preparation, namely sodium hydroxide solution and aniline, can also likewise contribute to the phase separation problem and/or mulm formation. Furthermore, it is not described in the two patent applications whether the problems of phase separation can be solved by measures in the work-up.
If the formation of a mulm layer cannot be avoided completely, it will ultimately pass into one of the two phases. If the mulm layer enters the organic phase, this is less acceptable in the case of phase separation after the neutralization of the crude product than in the case of phase separation after the washing of the neutralized product. This is because in the case of the last-mentioned phase separation, not only do relatively large amounts of water, but naturally also the substances dissolved therein, such as e.g. NaOH and NaCl, then pass together with the mulm layer from the neutralization into the further processing steps where they may be troublesome, e.g. as a result of salt deposits in apparatuses and pipelines in the area of distillation. Even if the formation of a mulm layer (e.g. as a result of the measures described in EP 1 652 835 A1 and WO 2008/148631 A1) can be prevented, the organic phase obtained after phase separation can still comprise considerable fractions of aqueous constituents as disperse phase which, in further processing steps, can result in similar problems to an entrained mulm layer.
EP 2 103 595 A1 discloses in connection with the phase separation after the neutralization of the crude product that this phase separation can be assisted by adding water and/or aniline. Preferably, the reaction mixture diluted by adding water and/or aniline is separated into an organic and aqueous phase in Florentine flasks having plate packs supporting the coalescence of the two phases as internals (paragraphs [0043] and [0044]). In the case of phase separation after the washing of the neutralized product, with this procedure it is not possible to achieve completely satisfactory results because here, as explained above, the requirements placed on the quality of the phase separation are much higher. This was not acknowledged in EP 2 103 595 A1.
It would therefore be desirable to provide processing measures in order to be able to overcome this problem.
The quality of a work-up process for the preparation of MDA is defined on the one hand by the content in the product of undesired impurities which arise due to inappropriate purification steps. On the other hand, the quality of a work-up process is defined by the fact that the overall process can be operated without technical production failure.
Although the described processes of the prior art succeed in preparing MDA with a high yield, no technical auxiliaries are described which could improve the separation off of the aqueous constituents from the washed neutralization product with the desired effectiveness.
There was therefore a need for a process for the preparation of di- and polyamines of the diphenylmethane series in which it is possible, as a result of simple measures, to minimize the aqueous fractions of the organic phase obtained after phase separation of the washed crude MDA. This would improve the cost effectiveness of existing MDA processes.